The present invention relates to operation of an auxiliary gas turbine as used for starting an aircraft engine and for driving other, on-board auxiliary equipment, such as an electric generator, hydraulic pumps, fuel pumps, etc. More particularly the invention relates to equipment for operating such a gas turbine under conditions which do not permit sustaining regular combustion for reasons of lack of adequate air supply.
Present day aircraft are usually equipped with a gas turbine for starting the engines without requiring assistance from ground equipment. However, the turbine is used also for driving auxiliary, secondary power generating and supply equipment, such as hydraulic pumps, generators etc. when the aircraft engines are off. Moreover, it was found advisable to have two power supply channels on board, one being driven by one of the engines and the second one by the gas turbine. This is particularly of advantage in the case of engine failure as the alternative power supply channel, driven by the gas turbine, remains functioning. In the case of a VTOL aircraft, the one channel is driven by a forward thrust producing engine; if it fails, the aircraft can still operate with the lift producing engines while the gas turbine drives the second power supply channel.
The operation of a gas turbine is not always without problems. When run by regular combustion, an adequate amount of air is needed to sustain combustion. Since low air pressure at high altitudes precludes such supply, it has been suggested to tap the compressor of an engine and to draw air therefrom as that compressor is sufficiently powerful. This is an adequate solution to the problem but fails when the particular engine drops out.
As an alternative solution to the problem of driving a gas turbine when insufficient air is present for combustion, it has been suggested to drive the gas turbine with gas developed, e.g., by decomposing hydrazine. The gas generated in a hydrazine decomposing reaction chamber will be fed directly to a turbine stage of the auxiliary gas turbine. The problem exists here that the thermodynamic conditions of operating the gas turbine with combustion gases or with hydrazine decomposition gases differ substantially.